Fiber optic gyroscopes employing the Sagnac effect to sense inertial rotation are well known. The inertial rotation induces a phase shift between two optical beams. The amount of phase shift is sensed by a photodetector, which provides a measurement of the inertial rotation rate. Designers of precision gyroscopes desire a method to induce electronically a phase shift between the two optical beams which is equal and opposite to the phase shift induced by the inertial rotation. The net phase shift sensed by the photodetector is then zero, and its output can be monitored with high gain amplifiers without concern for saturating the electronics. The amount of electronically induced phase shift required to null the photodetector output becomes the measurement of inertial rotation.
A common procedure to introduce an optical phase shift is for the electronics to produce a ramp voltage which is converted by a transducer to a ramp optical phase at one point within the fiber optic gyro. One optical beam will be modulated by the ramp before passing through the fiber optic ring; the other beam will be modulated after passing through the fiber optic ring. When the phases of the two optical beams are compared at the photodetector, one beam will contain the value of the phase ramp delayed in time with respect to the other beam. The time delay is the time, .tau., required for light to travel though the fiber optic ring, typically a few microseconds. The optical phase shift, .phi., is the slope of the ramp phase multiplied by the time delay, .tau.. By adjusting the slope of the ramp, the electronics can introduce a range of desired phase shifts and null the phase shift induced by the inertial rotation. The slope of the ramp is proportional to the inertial rate.
There is an obvious drawback to applying the ramp phase: The phase is proportional to the applied voltage, so the voltage would have to grow without bound, which is impractical. To circumvent this problem, the electronics will drive the ramp in voltage rapidly back to zero when the voltage reaches a certain level. The ramp immediately begins again. Hence, the voltage will produce a ramp of the appropriate slope almost all of the time, interspersed with rapid flybacks.
One problem that arises is at what voltage level the flyback to zero should be initiated. Preferably when the voltage reaches a level which corresponds to phase shift of 2.pi. radians, flyback should occur. The phase shift observed by the photodetector is contained within a trigonometric function and thus steps of 2.pi. radians do not change the output of the photodetector. Specifying the exact voltage level which corresponds to 2.pi. radians is problematical. While the gain of the transducer, in radians/volts, is known approximately, it can drift with time and temperature. Also, the gain of the transducers in different gyros will be slightly different. There is another practical complication: the flyback will always require some small but non-zero amount of time. The slope of the ramp is the critical quantity. To maintain the proper slope while allowing a small time for flyback, the voltage must flyback at a level slightly before reaching 2.pi. radians, so that it begins the next ramp at zero radians right at the time it would have reached a height of 2.pi. radians. In addition, as the slope of the ramp changes in response to changes in the rate of inertial rotation, the voltage level at which flyback should occur changes slightly.
Various techniques have been developed to address this problem. One approach uses a complex scheme for applying squarewave modulation of the optical beam. The amplitude of the modulation is switched between two levels depending on the height of the phase ramp. The squarewave modulation and the phase ramp are combined in the electronics so that a staircase of phase, rather than a continuous analog ramp, is applied to the optical beam. By adding some amplifiers to the basic control circuitry it is suggested that a means of adjusting the peak voltage level of the ramp to the proper value is achieve.
In another approach additional optics and one or more additional photodetectors are included in the fiber optic gyroscope. The optics are positioned to allow measurement of the phase shift induced by the ramp of phase in isolation from other phase shifts. The ramp of phase generates a sinusoid output from the photodetector. When the sinusoid has completed one full cycle, the height of the ramp is known to be 2.pi. and flyback is initiated.
Another scheme uses the high frequency phase modulation present in all precision fiber optic gyroscopes. The optical beams are typically phase modulated by a sinusoid of frequency f.sub.m to increase the sensitivity of the photodetector output to the phase shifts induced by the inertial rotation. The signal is then demodulated at f.sub.m to recover a voltage proportional to the net phase shift: the phase shift due to inertial rotation added to the phase shift due to the applied phase ramp. The phase modulator is calibrated by demodulating the output of the photodetector at 2f.sub.m and adjusting the amplitude of the sinusoidal phase modulation until the signal at the output of the demodulator at 2f.sub.m is driven to null. This will happen only if the amplitude of the sinusoid modulation is at a precise value, approximately 2.57 radians. Therefore, a means is provided for continuous measurement of the gain of the optical transducer, in radians/volt. If the same transducer is used for the phase ramp, then the electronics can scale the voltage required to produce 2.57 radians up to the voltage required to produce 2.pi. radians. When the voltage ramp reaches this level, flyback is initiated.